Production of meso-lactide, d-lactide and l-lactide by back biting of polylactide

ABSTRACT

Process for increasingly producing D-Lactide and meso lactide by depolymerizing by back biting polylactide (PLA) said process which comprises:
         (i) Depolymerizing polylactide into its corresponding dimeric cyclic esters by heating the polylactide in the presence of a catalyst system comprising a catalyst and a co-catalyst in a reaction zone at temperature and pressure at which the polylactide is molten;   (ii) Forming a vapor product stream from the reaction zone;   (iii) Removing the vapor product stream and optionally condense it;   (iv) Recovering, either together or separately meso-lactide, D-lactide and L-lactide.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation U.S. patent application Ser.No. 15/102,825, filed Jun. 8, 2016, which is a national stage entryunder 35 U.S.C. §371 of PCT/EP2014/077077, filed Dec. 9, 2014, whichclaims the benefit of European Application No. 13196418.1, filed Dec.10, 2013 the entire teachings and disclosures of which are incorporatedherein by reference thereto.

FIELD OF THE INVENTION

The present invention relates to a recycling depolymerization process(also called back biting process) wherein high molecular weightaliphatic polyester or co-polyester and particularly solid polylactide,oligomers of polylactide are recycled for depolymerization and to therecovery of the hydrocarboxylic values thereby in the form of cyclicesters.

More particularly the process of the invention is an improveddepolymerization process to selectively produce meso-lactide, andD-lactide in addition to the expected L-lactide.

BACKGROUND OF THE INVENTION

Within the frame of the production of polylactide (PLA) which is abiodegradable polymer, it becomes more and more important to be able toprepare copolymers with different contents of D-lactide.

While it is known that D-lactide may currently be obtained byfermentation of lactic acid of the D-configuration, it is also possibleto recover D-lactide or meso-lactide starting from PLA after adepolymerization treatment of it.

It is therefore of importance to develop a new way, rather than,starting with lactic acid, to increasingly produce D-lactide andmeso-lactide in order to prepare PDLA or copolymers containing both Land D-lactic acid enantiomer to form copolymers having differentproportions of D and L units.

There is thus a need for such a process.

Normally and conveniently, the polymer or mixture of polymers subject tothe depolymerization process comprises higher polymeric polylactide andis heated to produce cyclic esters. Such a process is generally veryslow. On top of that those depolymerization processes are focused moreon the purity of the lactide to recover than on a racemization toproduce the other enantiomer.

WO2014/000277 discloses a method for using a recycled polylactic acid toprepare lactide. The method comprises a) crushing recycled polylacticacid, b) melting and extruding the crushed recycled polylactic acid andentering the melt into a pre-depolymerization reactor to obtain apolylactic acid melt, c) making said melt undergo a chain scissionreaction at 180° C. to 250° C. with a catalyst to break the molecularchain of the melt (Mn<5000), d) making the polylactic acid melt withbroken chains undergo a depolymerization reaction at 150° C. to 250° C.with the degree of vacuum being −0.1MPa to −0.09 MPa, to produce crudelactide, e) separating the crude lactide through melt crystallization toobtain refined lactide. The content of the lactide can reach higher than99.5% and the optical purity can reach higher than 99.9%.

The Chemical and Pharmaceutical bulletin, Pharmaceutical Society ofJapan, vol. 47, no 4, 1 Apr. 1999, pages 467-471 discloses a series ofAl, Ti, Zn and Zr compounds which were evaluated as intramoleculartransesterification catalysts for the thermal depolymerization reactionof poly(L-lactic acid) oligomer resulting in LL-lactide, meso-lactideand DD-lactide.

EP 2 607 399 discloses a process for producing polylactic acid andreactors for use in said process. Example 2 refers to a depolymerizationprocess of polylactic acid oligomer with tin oxide catalyst.

US 2013/0023674 provides a method for the production of lactide directlyfrom recycled PLA wastes using thermal depolymerization process. Suchdepolymerization was exemplified in the presence of Ti (II) catalyst.The product recovered was mainly L-lactide.

Different patents, like Japanese Patent 2008-201679, also relate to theuse of a catalytic system comprising in addition to the catalyst an acidcompound, without other restriction, in order to improvedepolymerization.

A need exists therefore for an improved process for depolymerizingpolylactide, particularly and selectively to cyclic esters, i.e.,D-lactide, meso-lactide and L-lactide.

SUMMARY OF THE INVENTION

The present invention provides for a process for depolymerizing (by backbiting) polylactide (PLA) into its corresponding dimeric cyclic estersand increasingly producing D-lactide and meso-lactide, said processwhich comprises

-   -   (i) Depolymerizing the polylactide (PLA) into its corresponding        dimeric cyclic esters by heating the polylactide in the presence        of a catalyst system in a reaction zone at temperature and        pressure at which the polylactide is molten;    -   (ii) Forming a vapor product stream from the reaction zone;    -   (iii) Removing the vapor product stream and optionally condense        it;    -   (iv) Recovering either together or separately meso-lactide ,        D-lactide and L-lactide.

When used in the present invention, the terms increasingly producing aremeaning the production of an increased amount of D-lactide andmeso-lactide over that generally recoverable with usual process, andparticularly when starting with polylactide prepared from lactic acidhaving from 80% to 98.5% by weight of L-units and/or even withpolylactide prepared from lactic acid having more than 98.5% by weightof L-units. Such amount may reach up to 35% by weight of D-lactide andmeso-lactide, or even more (i.e. 55% by weight, when starting withlactic acid having a L-enantiomer content not exceeding 80% by weight).

According to the process of the present invention, to increasinglyproduce D-lactide and meso-lactide from depolymerization of polylactideis to heat said polylactide in a reaction zone, at a temperaturecomprised between 200 and 290° C., and preferably between 210 and 260°C., under reduced pressure, in the presence of a catalyst system, soenabling the formation of a vapor product stream containing lactidewhich is then distilled off

The polyester, co-polyester to be depolymerized in accordance with theprocess of the invention is an aliphatic polyester, or co-polyester,particularly polylactide, prepared from lactic acid having from 80% to98.5% by weight of L units, and/or polylactide, prepared from lacticacid having more than 98.5% by weight of L-units; said polyesters arecharacterized by a weight average molecular weight (Mw) lying between500 and 500,000 Dalton, particularly 30,000 and 300,000 Dalton, morepreferably 80,000 and 200,000 Dalton and having a ratio of the weightaverage molecular weight (Mw) to the number average molecular weight(Mn) called molecular weight distribution (MWD) of less than 2.0. Theweight average and number average molecular weight were measured bychromatography by gel permeation compared to a standard polystyrene inchloroform at 25° C. Measurement of the molecular masses may beperformed at 25° C. using a liquid chromatograph WATERS 610. Firstly, apolymer solution is prepared in chloroform (1 mg polymer/ml). Then, 100μl of this solution is taken and injected, through a filter with poresof 0.2 μm diameter, on the chromatograph column at 25° C. Molecularmasses are determined from the retention time in the column, translatedin mass equivalent using a universal calibration law based onpolystyrene standards. For example, ASTM practice D3016-97(2010) may beused.

The catalyst system to be used in the process of the present inventiongenerally comprises a catalyst and a co-catalyst; the catalyst isgeneral formula (M)(X1, X2, . . . Xm)n where M is a metal selected fromthe group comprising the elements of columns 3 to 12 of the periodictable of the elements as well as the elements Al, Ga, In, Ti, Ge, Sn,Pb, Sb, Bi, Ca and Mg, preferably from Sn, Zn and Mg and X1, X2 . . . Xmare each substituents selected from one of the classes of alkyls, aryls,oxides, carboxylates, halogenides, alkoxides as well as elements ofcolumns 15 and/or 16 of the periodic table,

m

is an integer ranging from 1 to 6 and

n

is an integer ranging from 0 to 6 ; the co-catalyst is selected from thegroup comprising (a) organic or inorganic acids having a dissociationconstant, pKa, around that of lactic acid which is for information of3.86, and particularly comprised between 3.0 and 4.7, preferably between3.5 and 4.2, more preferably between 3.7 and 3.9; (b) a compound ofgeneral formula (Y)(R1, R2, . . . , Rq)p, where Y is an element selectedfrom the group comprising the elements of columns 15 or 16 of theperiodic table of the elements and particularly Phosphorus, and whereR1, R2, . . . Rq are each substituents selected from one of the classesof alkyls, aryls, oxides , halogenides, oxyalkyls, aminoalkyls,thioalkyls, phenoxides, aminoaryls, and thioaryls,

q

is an integer ranging from 1 to 6 and

p

0 is an integer ranging from 0 to 6; and (c) an organosilane aliphaticor cycloaliphatic selected from the group comprising alkyl alkoxysilaneor the cycloalkylalkoxysilane represented by the general formulaQQ′Si(O-alkyle)₂, where the Q and Q′ are the same or different and arealkyle or cycloalkyle radical containing from 1 to 8 carbon atoms.

Among the cocatalysts selected from the group comprising organic orinorganic acids one can cite for example the fumaric acid, the lacticacid and the glycolic acid.

Among the cocatalysts selected from the group comprising a compound ofgeneral formula (Y)(R1, R2, . . . , Rq)p one can cite for exampletriphenylphosphine.

Among the cocatalysts selected from the group comprising an organosilanealiphatic or cycloaliphatic one can cite for exampledicyclopentyldimethoxysilane, cyclohexylmethyldimethoxysilane,diisobutyldimethoxysilane and t-butylethyldimethoxysilane.

The catalyst of the catalyst system is used in an amount comprisedbetween 0.05 and 3% by weight of the PLA, preferably between 0.1 and 2%by weight and the co-catalyst is generally used in an amount comprisedbetween 0.1 and 10.0% and preferably between 1.0 and 8.0% by weight ofthe PLA.

In the present invention the main ingredient of the feed to be treatedby depolymerisation is PLA and it represents at least 60% by weight,preferably 80% by weight of the feed to depolymerize.

For the present invention PLA means polylactide or polylactic acid andrepresents poly-L-lactic acid (PLLA), poly-D-lactic acid (PDLA) andpolylactic acid of racemic L- and D-; it also comprises polylactic acidstereocomplex as well as PLA-urethane copolymers.

In carrying out the process of the invention, the feed containing thePLA is first subject to a thermal decomposition at temperature comprisedbetween 200 and 290° C., preferably between 210 and 260° C. and eitherat a reduced pressure generally under lactide vapour pressure, or bycarrying away with an inert gas like nitrogen, and in the presence of acatalytic system.

The Applicants have now found that to favorize the production ofD-lactide and meso-lactide, the temperature has to be controlledcarefully. Indeed, below 200° C., when applying the process of theinvention in the presence of the catalyst system, the final yield inD-lactide and meso-lactide is very low and therefore does not presentinterest. It may be said, that even for the recovery of L-lactide theyield is not significantly high.

On the other hand, when using temperatures higher than 290° C., thereappears a certain degradation of the products, so lowering drasticallythe yield in D-lactide and meso-lactide.

It is therefore unexpected that in a certain range of temperatures inthe depolymerization step, in combination with the use of theco-catalyst, the Applicants obtained so high production of D-lactide andmeso-lactide, even when starting with PLLA for which their content inthe starting material is very low.

According to an embodiment of the process of the invention, the mixtureof polymers containing PLA is therefore introduced in a reaction zonewhere it is heated to depolymerize. The vapors formed during saidreaction, or back biting, are then condensed and then recoveredseparately. The duration of the heating step will depend of the reactiontemperature.

The catalytic system is introduced together with the mixture of polymersin the reaction zone and comprises any catalyst enabling the thermolysisof PLA into cyclic esters. Suitable catalysts are generally metals orcompounds of metals of columns 3 and 12 of the periodic table of theelements and of Mg and Sn; typical catalysts in the catalyst system arepreferably selected from the group comprising Sn(II) carboxylates likeSn octanoate, MgO, ZnO, or even a mixture thereof

According to the process of the present invention, the Applicant havenow found interesting results are obtained to produce selectivelyD-lactide and meso-lactide even when starting with PLLA (more than 98.5%by weight of L-configuration) rather than with normal polylactide(between 80 and 90% by weight of L-configuration), if a suitablecombination of the depolymerization temperature is applied together withthe presence of a co-catalyst selected from either an organic orinorganic acid having a dissociation constant, pKa, ranging between 3.0and 4.7, preferably between 3.5 and 4.2 and more preferably between 3.7and 3.9, or a phosphorus compound, preferably triphenylphosphine, or analkoxysilane derivatives, the depolymerization temperature rangingbetween 210° C. and 260° C. Typical depolymerization times may last froma few minutes i.e 10 min to several hours i.e 3 h depending on thedepolymerization reactor used.

The vapor product stream exiting the reaction zone, normally comprisesthe dimeric cyclic ester, this means L-lactide, D-lactide andmeso-lactide but also other volatiles components. The vapor product isthen optionally subjected to a condensation step or further processed assuch. The condensed vapor products, if any, are readily separated, byany suitable means in its constituents (e.g. distillation,crystallization . . . ).

The Applicants have also found that the association of the catalyst withthe co-catalyst has an importance on the racemization of the lactide.Indeed, when using a catalyst system implementing Sn octanoate ascatalyst it is preferable to use acids, more preferably organic acids asco-catalyst, having a pKa>4, such as for example fumaric acid(pKa₂:4.44) but among the catalyst system with Sn octanoate as catalystthe preferred co-catalyst is triphenylphosphine (TPP), while with MgO orwith a mixture of MgO and ZnO as catalyst the co-catalyst is preferablyselected from organic acids having a pKa comprised between 3.0 and 4.0like lactic acid (pKa:3.86) or glycolic acid (pKa: 3.88).

When MgO is used as catalyst, one can also use as cocatalyst anorganosilane aliphatic or cycloaliphatic selected from the groupcomprising alkylalkoxysilane or the cycloalkylalkoxysilane representedby the general formula QQ′Si(O-alkyle)₂, where the Q and Q′ are the sameor different and are alkyle or cycloalkyle radical containing from 1 to8 carbon atoms. Among these cocatalysts, cycloalkylalkoxysilane ispreferably used such as for example cyclohexylmethyldimethoxysilane.

Another advantage of the process of the invention is to produce a muchmore important proportion of D-lactide and meso-lactide during thedepolymerization, particularly when the starting material is lactic acidconstituted of substantially completely of enantiomer L.

It has been noted that when the process of the invention is integratedin a global process starting with lactic acid to obtain a polymerthereof, it is now feasible to consider that said global process mayalso integrate the preparation of copolymers containing both enantiomersL and D, by simply introducing D-lactide and meso-lactide together withthe L-lactide into the ring opening polymerization reactor.

The process of the invention enables to manufacture any type of in situcopolymers of P(L-D)LA and particularly those whose enantiomer D contentbeing such it may reach 30-35% by weight, while it was not easilyfeasible in such a way in the prior art processes, except by blends.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is representing a process to prepare PLA starting with lacticacid containing more than 98.5% by weight enantiomer L, said processstarting with an aqueous solution of lactic acid from which water isevaporated to obtain lactic acid which is then prepolymerized andsubject to cyclization to form crude L-lactide which needs to bepurified before being polymerized by ring opening of lactide, andfinally the resulting PLA is purified by devolatilization.

DETAILED DESCRIPTION OF THE INVENTION

According to the process of the present invention, it is now possible tointroduce, at the level of the purification of the crude lactide, astream either liquid or gaseous, of a mixture rich in meso-lactide andD-lactide resulting from the depolymerization by back biting; saidstream rich in meso-lactide and D-lactide is mixed with the stream ofL-lactide and both streams are subject together to the ring openingpolymerization, in accordance with usual conditions generally appliedfor such reaction.

So, a copolymer is now formed containing both enantiomers L and D-.These latter being in a proportion which may reach 30 to 35% by weight.

It is understood that lower contents may also be reached, simply bycontrolling the feeding of the back biting stream in the main stream ofcrude L-lactide to be purified.

Another advantage of the proces of the invention, based on the importantproportion of D-lactide and meso-lactide produced during thedepolymerization, even if the starting material is constitutedsubstantially completely of enantiomer-L, is to envision an integratedprocess for the production of homopolymers or copolymers of PLA, i.e.,PLLA or PLLA-PDLA, starting with an aqueous solution of lactic acid notcontaining more than 10% of D-lactic acid.

Such an integrated process is generally consisting of the removal ofwater from the starting lactic acid aqueous solution, theoligomerization of lactic acid into oligomers of 400 to 5,000 Dalton,the cyclisation of said oligomers to produce crude lactide then subjectthe obtained crude lactide to purification, and finally thepolymerization by ROP of the lactide into PLA.

Owing to the process of the invention which can produce more D-lactideand meso-lactide by depolymerization of PLA waste and off spec of theintegrated process, this stream may be introduced directly in thepurification step of the lactide of the integrated process, according towhich the amount of D-lactide and/or meso lactide introduced iscontrolled in order to achieve the desired composition of copolymers.

Generally, the composition of the copolymer may contain up to 25 to 35%by weight of D-lactide and/or meso-lactide.

It is agreed that the process of the invention is not limited to such anintegrated process, particularly when the feed of the back biting stepis comprising waste of PLA coming from other sources than the presentprocess; PLA waste from such sources may of course contain differentamount of D-enantiomer which may reach 20% by weight;

EXAMPLES

The back biting of various PLA samples, all having an enantiomer Lcontent of 99.4% by weight has been carried out.

First the samples were ground and then deposited into a recipient (thereaction zone) which was introduced into the reactor. Then the catalystand co-catalysts were added in the reaction zone. The Sn octanoate (Sn(oct)₂) used as catalyst was added in an amount of 1% by weight of thePLA. Regarding the co-catalyst, when fumaric acid was used (pKa₁=3.03;pKa₂=4.44), it was added in an amount of 5% by weight of the PLA. Whentriphenylphosphine was used as co-catalyst, it was added in an amount of0.7% by weight of the PLA.

The temperature was then raised up to 250° C. and maintained during 60minutes.

The pressure was adjusted to 10 millibar.

During the period of time the reaction mixture was maintained at saidtemperature, a vapor product was formed and further extracted from thereaction zone while the vapor product was then subject to a condensationstep.

The condensed product was recovered and analysed by gas chromatography(GC) to determine its constituants and their respective contents in % byweight in L-lactide, D-lactide and meso-lactide. The lactide yield (%)represents the quantity of lactide recovered and condensed.

The results are presented here below.

lactide L- D- Meso- Impur- yield lactide lactide Lactide ities Ex.Catalyst Co-Cata (%) (%) (%) (%) (%) 1 Sn (oct)₂ None 90.7 83.3 1.7 14.01.0 2 Sn (oct)₂ Fumaric 65.0 68.0 6.0 25.0 1.0 Acid 3 Sn (oct)₂ TPP 83.868 8.5 22.3 0.2

A comparative example was conducted under the same conditions as thosedescribed for the examples according to the invention excepted that thesulfamic acid having a pKa of 0.99 was used as co-catalyst at 5.3% byweight of the PLA.

lactide L- D- Meso- Impur- Yield lactide lactide lactide ities Ex.Catalyst Co-Cata (%) (%) (%) (%) (%) 4 Sn (oct)₂ Sulfamic 1.1 95 None0.9 4.1 Acid

Other examples within the process of the invention were conducted underthe same operating conditions as those described above excepted thatanother catalyst than Sn octanoate and cocatalyst were used. MgO wasused as catalyst in an amount of 1% by weight of the PLA and lactic acid(pKa: 3.86) was used as cocatalyst, it was added in an amount of 5% byweight of the PLA (see example 5). In example 6, MgO was used ascatalyst in an amount of 1% by weight of the PLA andcyclohexylmethyldimethoxysilane was used as cocatalyst in an amount of5% by weight of the PLA. In example 7, the same example was conductedwithout any cocatalyst (comparative example).

lactide L- D- Meso- Impur- Yield lactide lactide lactide ities Ex.Catalyst Co-Cata (%) (%) (%) (%) (%) 5 MgO Lactic Acid 81.0 70.3 11.018.2 0.5 6 MgO cyclohexyl- 90.0 37.4 30.7 28.5 4.4 methyl dimethoxy-silane 7 MgO None 60.9 92.3 1.4 6.1 0.2

What is claimed is:
 1. Process for increasingly producing D-Lactide andmeso lactide by depolymerizing by back biting polylactide (PLA) saidprocess which comprises: (i) Depolymerizing polylactide into itscorresponding dimeric cyclic esters by heating the polylactide in thepresence of a catalyst system which comprises a catalyst and aco-catalyst, the catalyst of general formula (M)(X1, X2, . . . Xm)nwherein M of the catalyst is selected from the group consisting of Sn,Zn and Mg, and X1, X2, . . . Xm are each substituents selected from oneof the classes of alkyls, aryls, oxides, carboxylates, halogenides,alkoxides as well as elements of columns 15 and/or 16 of the periodictable,

m

is an integer ranging from 1 to 6 and

n

is an integer ranging from 0 to 6 ; the co-catalyst is selected from thegroup comprising an organosilane aliphatic or cycloaliphatic selectedfrom the group comprising alkylalkoxysilane or thecycloalkylalkoxysilane represented by the general formulaQQ′Si(O-alkyle)₂, where the Q and Q′ are the same or different and arealkyle or cycloalkyle radical containing from 1 to 8 carbon atoms, in areaction zone at temperature and pressure at which the polylactide ismolten; (ii) Forming a vapor product stream from the reaction zone;(iii) Removing the vapor product stream and optionally condense it; (iv)Recovering, either together or separately meso-lactide, D-lactide andL-lactide.
 2. Process according to claim 1 wherein the catalyst is Snoctanoate, MgO, ZnO or a mixture thereof
 3. Process according to claim 1wherein the catalyst system is constituted of MgO as catalyst and acycloalkylalkoxysilane represented by the general formulaQQ′Si(O-alkyle)₂, where the Q and Q′ are different and are alkyle orcycloalkyle radical containing from 1 to 8 carbon atoms as cocatalyst.4. Process according to claim 3 wherein the catalyst system isconstituted of MgO as catalyst and cyclohexylmethyldimethoxysilane ascocatalyst.
 5. Process according to anyone of claim 1 wherein thecatalyst of the catalyst system is used in an amount comprised between0.05 and 3% by weight of PLA and the co-catalyst is used in an amountcomprised between 0.1 and 10.0% by weight of PLA
 6. Process according toclaim 1 wherein the depolymerization is carried out at a temperaturebetween 200 and 290° C. and at a pressure under lactide vapour pressure.7. Proces of mixing D-lactide and meso lactide obtained according toclaim 1 with L-lactide to prepare a copolymer of P(L-D)LA by ringopening polymerization, the content of enantiomer D- of said copolymernot exceeding 35% by weight.
 8. Process according to claim 6 wherein thedepolymerization is carried out at a temperature between 210 and 260° C.and at a pressure under lactide vapour pressure.
 9. Process forincreasingly producing D-Lactide and meso lactide by depolymerizing byback biting polylactide (PLA) said process which comprises: (i)Depolymerizing polylactide into its corresponding dimeric cyclic estersby heating the polylactide in the presence of a catalyst system whichcomprises a catalyst and a co-catalyst, wherein the catalyst is Snoctanoate, MgO, ZnO or a mixture thereof ; the co-catalyst is selectedfrom the group comprising an organosilane aliphatic or cycloaliphaticselected from the group comprising alkylalkoxysilane or thecycloalkylalkoxysilane represented by the general formulaQQ′Si(O-alkyle)₂, where the Q and Q′ are the same or different and arealkyle or cycloalkyle radical containing from 1 to 8 carbon atoms, in areaction zone at temperature and pressure at which the polylactide ismolten; (ii) Forming a vapor product stream from the reaction zone;(iii) Removing the vapor product stream and optionally condense it; (iv)Recovering, either together or separately meso-lactide, D-lactide andL-lactide.
 10. Process according to claim 9 wherein the catalyst systemis constituted of MgO as catalyst and a cycloalkylalkoxysilanerepresented by the general formula QQ′Si(O-alkyle)₂, where the Q and Q′are different and are alkyle or cycloalkyle radical containing from 1 to8 carbon atoms as cocatalyst.
 11. Process according to claim 10 whereinthe catalyst system is constituted of MgO as catalyst andcyclohexylmethyldimethoxysilane as cocatalyst.
 12. Process according toanyone of claim 9 wherein the catalyst of the catalyst system is used inan amount comprised between 0.05 and 3% by weight of PLA and theco-catalyst is used in an amount comprised between 0.1 and 10.0% byweight of PLA
 13. Process according to claim 9 wherein thedepolymerization is carried out at a temperature between 200 and 290° C.and at a pressure under lactide vapour pressure.
 14. Proces of mixingD-lactide and meso lactide obtained according to claim 9 with L-lactideto prepare a copolymer of P(L-D)LA by ring opening polymerization, thecontent of enantiomer D- of said copolymer not exceeding 35% by weight.15. Process according to claim 13 wherein the depolymerization iscarried out at a temperature between 210 and 260° C. and at a pressureunder lactide vapour pressure.